High performance microwave GaN-on-SiC HEMTs are demonstrated on a heterostructure without a conventional thick doped buffer. The HEMT is fabricated on a high-quality <inline-formula> <tex-math ...notation="LaTeX">0.25~\boldsymbol {\mu }\text{m} </tex-math></inline-formula> unintentional doped GaN layer grown directly on a transmorphic epitaxially grown AlN nucleation layer. This approach allows the AlN-nucleation layer to act as a back-barrier, limiting short channel effects and removing buffer leakage. The devices with the 'buffer-free' heterostructure show competitive DC and RF characteristics, as benchmarked against the devices made on a commercial Fe-doped epi-wafer. Peak transconductances of 500 mS/mm and a maximum saturated drain current of ~1 A/mm are obtained. An extrinsic <inline-formula> <tex-math notation="LaTeX">\text{f}_{\sf T} </tex-math></inline-formula> of 70 GHz and <inline-formula> <tex-math notation="LaTeX">\text{f}_{\sf max} </tex-math></inline-formula> of 130 GHz are achieved for transistors with a gate length of 100 nm. Pulsed-IV measurements reveal a lower current slump and a smaller knee walkout. The dynamic IV performance translates to an output power of 4.1 W/mm, as measured with active load-pull at 3 GHz. These devices suggest that the 'buffer-free' concept may offer an alternative route for high frequency GaN HEMTs with less electron trapping effects.
An orthogonal vector approach is proposed for the synthesis of multi-beam directional modulation (DM) transmitters. These systems have the capability of concurrently projecting independent data ...streams into different specified spatial directions while simultaneously distorting signal constellations in all other directions. Simulated bit error rate (BER) spatial distributions are presented for various multi-beam system configurations in order to illustrate representative examples of physical layer security performance enhancement that can be achieved.
Metal organic frameworks (MOFs) have attracted great interest in photocatalysis, but their activity is hampered by the issue of severe carrier recombination. Here, through a carboxyl group assisted ...coordination route, MXene decorated with carboxyl groups provides chelation sites enabling coordination with UiO‐66‐NH2(Zr/Ti) (UZT) to fabricate a tightly connected UiO‐66‐NH2(Zr/Ti)/carboxyl‐functionalized MXene (UZT/CFMX) heterostructure. This is the first instance of direct chemical bonding of MOFs‐involved heterostructure via a coordination bond. The critical role of decorated carboxyl groups can be determined so that 1) these can help to establish a strong coordination bond between two materials; 2) act as bridge to promote the electrons transfer from MOFs to MXene, thus relieving carrier recombination, and 3) most interestingly, the carbon atom on the carboxyl group forms a bond with the oxygen from water stimulating the water to dissociate into OH* and H*, thus adding additional reaction pathways for promoting photocatalytic water splitting. Accordingly, the resulting UZT/CFMX shows efficient solar‐driven photocatalytic performance for water splitting. The H2 evolution rate is as high as 2187 µmol g−1 h−1, 20 times higher than that of UZT and 4 times higher than that of UiO‐66‐NH2 (Zr/Ti)/MXene (UZT/MX), also surpassing the majority of reported MOF‐based photocatalysts.
Through a unique carboxyl group assisted coordination route, UiO‐66‐NH2(Zr/Ti)/carboxyl‐functionalized MXene (UZT/CFMX) composite with intimate interactions is successfully constructed. Such intimately coupled heterojunctions display high activity for solar‐driven water splitting.
The over 60 years old Rashba-Dresselhaus effect predicts spin-orbit coupling (SOC) induced momentum-dependent spin splitting and spin polarization in materials with noncentrosymmetric structures. ...Strong SOC induced effects usually require high-atomic number (Z) elements such as rare-earth elements. It has recently been pointed out that antiferromagnets could hold SOC-independent spin splitting and spin polarization. In the present work we develop the spatial and magnetic symmetry conditions enabling such antiferromagnet (AFM)-induced spin splitting, dividing the 1651 magnetic space groups into seven different spin splitting prototypes (SST-1 to SST-7). This analysis places the physics of AFM spin splitting (SST-4) within the broader context of symmetry conditions that enable the more familiar forms of spin splitting, such as ferromagnetic Zeeman effect (SST-5), nonmagnetic no spin splitting (SST-6), and the nonmagnetic Rashba and Dresselhaus effects (SST-7). The AFM-induced spin splitting and spin polarization do not necessarily require breaking of inversion symmetry or the presence of SOC, hence can exist even in centrosymmetric, low-Z light element compounds, considerably broadening the material base for spin manipulations. We use the “inverse design” approach of first formulating the target property (here, spin splitting in low-Z compounds not restricted to low symmetry structures), then derive the enabling physical design principles—the magnetic symmetry conditions—to search realizable compounds that satisfy these a priori design principles. This process uncovers 422 magnetic space groups (160 centrosymmetric and 262 noncentrosymmetric) that could hold AFM-induced, SOC-independent spin splitting and spin polarization. We then search for stable compounds following such enabling symmetries. We investigate the electronic and spin structures of some selected prototype compounds by density functional theory (DFT) and find spin textures that are different than the traditional Rashba-Dresselhaus patterns and exist even in the absence of SOC effect. We provide the DFT results for all antiferromagnetic spin splitting prototypes (SST-1, SST-2, SST-3, SST-4), and concentrate on two limits of SST-4 that are particularly unusual: When spin splitting is momentum dependent (just like the Rashba effect) but is enabled in antiferromagnets even in the absence of SOC in the Hamiltonian. This includes examples of (a) centrosymmetric SST-4A compounds (e.g., orthorhombic LaMnO3 illustrating collinear AFM, as well as cubic NiS2 illustrating noncollinear AFM) and (b) noncentrosymmetric SST-4B compounds (e.g., rhombohedral MnTiO3 illustrating collinear AFM and hexagonal ScMnO3 illustrating noncollinear AFM). The symmetry design principles outlined here, along with their transformation into an inverse design material search approach and DFT verification, could open the way to their experimental examination.
Alloy anodes have shown great potential for next‐generation lithium‐ion batteries (LIBs) and sodium‐ion batteries (SIBs). However, these applications are still limited by inherent huge volume changes ...and sluggish kinetics. To overcome such limitations, graphene‐protected 3D Sb‐based anodes grown on conductive substrate are designed and fabricated by a facile electrostatic‐assembling and subsequent confinement replacement strategy. As binder‐free anodes for LIBs, the obtained electrode exhibits reversible capacities of 442 mAh g−1 at 100 mA g−1 and 295 mAh g−1 at 1000 mA g−1, and a capacity retention of above 90% (based on the 10th cycle) after 200 cycles at 500 mA g−1. As for sodium storage properties, the reversible capacities of 517 mAh g−1 at 50 mA g−1 and 315 mAh g−1 at 1000 mA g−1, the capacity retention of 305 mAh g−1 after 100 cycles at 300 mA g−1 are obtained, respectively. Furthermore, the 3D architecture retains good structural integrity after cycling, confirming that the introduction of high‐stretchy and robust graphene layers can effectively buffer alloying anodes, and simultaneously provide sustainable contact and protection of the active materials. Such findings show its great potential as superior binder‐free anodes for LIBs and SIBs.
Using a facile electrostatic assembly and graphene‐confined replacement strategy, 3D Sb‐based anodes grown on nickel foam (NF) are designed and fabricated by utilizing NF as both reductant and substrate. When employed as a binder‐free anode for lithium and sodium storage, the prepared product shows superior rate capability and stable cyclability. In addition, the obtained 3D structure maintains good structural stability upon cycling.
A new synthesis of d-lyxose from d-arabinose Liang, Fa-Zhan; Chen, Yu; Pan, Xian-Hua ...
Carbohydrate research,
April 2023, 2023-Apr, 2023-04-00, 20230401, Letnik:
526
Journal Article
Recenzirano
A new synthesis of rare d-lyxose from easily available d-arabinose is disclosed. The route includes 7 steps with a total 40% yield. Inversion of configuration at C3 promoted by DAST reagent is ...utilized on trans-2,3-di-hydroxy pentofuranose to provide cis-2,3-di-hydroxy pentofuranose, which is hardly synthesized using normal method.
Display omitted
•Synthesis of rare d-lyxose from widely available d-arabinose.•Inversion of configuration promoted by DAST reagent.•Neighboring group participation by benzoyl group.•40% total yields using only 7 steps.
Rapidly decaying long-period oscillations often occur in hot coronal loops of active regions associated with small (or micro-) flares. This kind of wave activity was first discovered with the ...SOHO/SUMER spectrometer from Doppler velocity measurements of hot emission lines, thus also often called “SUMER” oscillations. They were mainly interpreted as global (or fundamental mode) standing slow magnetoacoustic waves. In addition, increasing evidence has suggested that the decaying harmonic type of pulsations detected in light curves of solar and stellar flares are likely caused by standing slow-mode waves. The study of slow magnetoacoustic waves in coronal loops has become a topic of particular interest in connection with coronal seismology. We review recent results from SDO/AIA and Hinode/XRT observations that have detected both standing and reflected intensity oscillations in hot flaring loops showing the physical properties (e.g., oscillation periods, decay times, and triggers) in accord with the SUMER oscillations. We also review recent advances in theory and numerical modeling of slow-mode waves focusing on the wave excitation and damping mechanisms. MHD simulations in 1D, 2D and 3D have been dedicated to understanding the physical conditions for the generation of a reflected propagating or a standing wave by impulsive heating. Various damping mechanisms and their analysis methods are summarized. Calculations based on linear theory suggest that the non-ideal MHD effects such as thermal conduction, compressive viscosity, and optically thin radiation may dominate in damping of slow-mode waves in coronal loops of different physical conditions. Finally, an overview is given of several important seismological applications such as determination of transport coefficients and heating function.
Controlling the structure of catalysts at the atomic level provides an opportunity to establish detailed understanding of the catalytic form-to-function and realize new, non-equilibrium catalytic ...structures. Here, advanced thin-film deposition is used to control the atomic structure of La
Sr
MnO
, a well-known catalyst for the oxygen reduction reaction. The surface and sub-surface is customized, whereas the overall composition and d-electron configuration of the oxide is kept constant. Although the addition of SrMnO
benefits the oxygen reduction reaction via electronic structure and conductivity improvements, SrMnO
can react with ambient air to reduce the surface site availability. Placing SrMnO
in the sub-surface underneath a LaMnO
overlayer allows the catalyst to maintain the surface site availability while benefiting from improved electronic effects. The results show the promise of advanced thin-film deposition for realizing atomically precise catalysts, in which the surface and sub-surface structure and stoichiometry are tailored for functionality, over controlling only bulk compositions.
Polysaccharide-based nanogels have drawn considerable interest in pharmaceutics because of their superior biocompatibility and potential responsiveness to external stimuli, enabling specific drug ...release. During the fabrication of nanogels, however, covalent cross-linking often involves less friendly cross-linkers and traditionally employed noncovalent cross-linking often relies on weak interactions that may lead to premature payload release. Herein, we report host–guest chemistry-driven supramolecular chitosan nanogels (CNGs) that are responsive to either endogenous or exogenous stimuli, thus allowing selective drug release in specific cancer cells or disease sites. In an aqueous solution, two phenylalanine (Phe) units of Phe-grafted chitosan (CS-Phe) were encapsulated into one cavity of cucurbit8uril (CB8), driving cross-linking of CS-Phe and formation of CNGs. Doxorubicin hydrochloride (DOX), a chemotherapeutic agent, was entrapped in the matrix of CNGs during the formation of nanogels to yield DOX–CNGs with an excellent drug loading efficiency. The morphology and size of CNGs were fully assessed by transmission electron microscopy and dynamic light scattering. The encapsulated DOX was selectively liberated in the presence of competitive guests of CB8, such as endogenous spermine (SPM) that is overexpressed by certain types of cancer cells or exogenous amantadine (ADA) that may be added into cells or tissues that require targeted treatment, either of which may replace Phe from the cavity of CB8 resulting in the breakdown of the nanogels and payload release. The CNGs were efficiently internalized by cells, and the DOX–CNGs exhibited specific, potent activity against cancerous cells such as A549 cell line that is well known for SPM overexpression. This study reports that the first stimuli (competitive guest)-responsive host–guest interactions initiated supramolecular CNGs with excellent biocompatibility and selective therapeutic efficacy against cancer cells. It may provide new insights into the design and fabrication of novel stimuli-responsive payload delivery systems.
Quasi-periodic pulsations (QPPs) are usually found in the light curves of solar and stellar flares; they carry the features of time characteristics and plasma emission of the flaring core, and could ...be used to diagnose the coronas of the Sun and remote stars. In this study, we combined the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory and the Nobeyama Radioheliograph (NoRH) to observe an M7.7 class flare that occurred at active region 11520 on 2012 July 19. A QPP was detected both in the AIA 131 bandpass and the NoRH 17 channel; it had a period of about four minutes. In the spatial distribution of Fourier power, we found that this QPP originated from a compact source and that it overlapped with the X-ray source above the loop top. The plasma emission intensities in the AIA 131 bandpass were highly correlated within this region. The source region is further segmented into stripes that oscillated with distinctive phases. Evidence in this event suggests that this QPP was likely to be generated by intermittent energy injection into the reconnection region.
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